Eight inches of soil really isn't deep enough to support the listed plants for any length of time. Rather than haul soil up to your rooftop garden, have you considered gardening in large containers? You can vary the sizes of containers, group them together and rearrange them at will, which will give your rooftop garden a different look at any given time. Eight inches of soil will support annual plants such as pansies, lobelia, and some bulbs, but not evergreen trees and shrubs.